بِالْحُسْنَى (The best reward): It is the Paradise. The Arabic word is a superlative tense noun. Allah (swt) says:

{ وَعَدَ اللَّهُ الْمُؤْمِنِينَ وَالْمُؤْمِنَاتِ جَنَّاتٍ تَجْرِي مِن تَحْتِهَا الْأَنْهَارُ خَالِدِينَ فِيهَا وَمَسَاكِنَ طَيِّبَةً فِي جَنَّاتِ عَدْنٍ ۚ وَرِضْوَانٌ مِّنَ اللَّهِ أَكْبَرُ ۚ ذَٰلِكَ هُوَ الْفَوْزُ الْعَظِيمُ}

(Allah has promised the believing men and believing women gardens beneath which rivers flow, wherein they abide eternally, and pleasant dwellings in gardens of perpetual residence; but approval from Allah is greater. It is that which is the great attainment.) (At-Tawbah: 72)

A promise is a glad tiding of good news in future as Allah (swt) has promised whoever follows His method by the best reward which is the Paradise, while the plus of that, according to what interpreters said, is to see the benefactor; Allah (swt).